During offshore drilling operation with a floating drilling vessel, the vessel is connected to the well head via the drilling riser. The vessel also experiences a heaving motion due to oceanic waves. This heaving motion puts additional stress into the riser and could potentially cause a catastrophic failure.
This problem of riser stress induced by heaving motion is currently solved by inserting a telescopic joint into the riser. The telescopic joint is a mechanism designed to continuously adapt the length of the riser during drilling operations to compensate for the horizontal and vertical displacements of the drilling vessel. To accomplish this, an outer barrel of the telescopic joint is fixed to the riser, and an inner barrel of the telescopic joint slides inside the outer one while the vessel heaves up and down due to wave motion. Such a telescopic joint is also referred to as a slip joint. The vessel is connected to the outer barrel using hydraulic or cable tensioners and a tension ring. The tensioners are used to maintain a nearly constant tension in the riser. A locking mechanism is also used with the telescopic joint in order to fix the inner barrel to the outer barrel during installation, maintenance, and abandonment. A more complete set of requirements for the telescopic joint can be found in API spec 16F, Specification for Marine Drilling Riser Equipment, first edition, August 2004, American Petroleum Institute, Washington, DC.
In existing applications, the telescopic joint has a rubber packer which, when activated by pressure from a pump, seals between the inner and outer barrels and allows the flow of drilling fluid without leakage from the riser as the drilling fluid returns from the well. In this type of design, the useful life of the rubber packer is limited by the wear due to the sliding action of the inner barrel. To extend the short life of such devices, a backup packer is installed, and the backup packer is activated after the first packer reaches the end of its useful life.
Examples of current commercial telescopic joints are the GE VetcoGray Telescopic Joint and the Cameron Telescopic RD Riser Joint. The GE VetcoGray Telescopic Joint is shown on page 14 of the GE Drilling Systems Brochure, No. 080709, 2009, GE Oil & Gas, Houston, Tex.
Currently the standard sizes of drilling risers used with these telescopic joints are 16″, 18⅝″, 20″, 21″, 22″, and 24″ (406.4 mm, 473.1 mm, 508 mm, 533.4 mm, 558.8 mm, 609.6 mm) in diameter. The inner diameter of the innermost barrel should be no less than the inner diameter of the mating riser pipe. The amount of stroke required for the telescopic joint is based on predicted wave patterns. Among the longer lengths of stroke is roughly 50 feet (15 meters). API spec 16F also lists tension load ratings up to 4 million pounds (17,800 kN). The operating pressures at the telescopic joint are low. The hydrostatic test requirement, per Section 11.6.2.1 of API 16F, calls for pressures of 25, 50, 100 and 200 psi (0.17, 0.34, 0.69, and 1.38 M Pa) to be sustained without leakage for no less than 15 minutes.